1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus that images the inside of a subject by using a magnetic resonance phenomenon, and a scanning-condition setting method. The present invention particularly relates to a technology for controlling a scanning parameter so as to make a Signal-to-Noise (SN) ratio of an image to be obtained through a scan constantly higher than a certain value.
2. Description of the Related Art
A magnetic resonance imaging method used by a magnetic resonance imaging apparatus is a method of acquiring chemical and physical microscopic information about a substance by using a magnetic resonance phenomenon. The magnetic resonance phenomenon is a phenomenon that when being placed in a magnetic field, an aggregation of subject nuclear spins resonates with a radio-frequency magnetic field in which each atomic nucleus spins at a particular frequency (resonance frequency) responding to its own unique magnetic moment and an existing magnetic field, and generates a signal (magnetic resonance signal) in a relaxation process.
According to the magnetic resonance imaging apparatus, the Signal-to-Noise (SN) ratio of an image, the spatial resolution, the scan time, or the like depends on, for example, the type of a pulse sequence or a scanning parameter of the pulse sequence, such as a Field Of View (FOV), a scan matrix, the number of acquisition (number of excitation), or a receiving band. For this reason, the MRI apparatus is an apparatus that needs to set especially various scanning conditions among medical devices.
The SN ratio of an image obtained by the magnetic resonance imaging apparatus has a large influence on the image quality of the obtained image. Sometimes an image of a low SN ratio brings about an obstacle to diagnosis, therefore, it needs to obtain an SN ratio constantly higher than a certain value. To secure a stable SN ratio, usually, scanning parameters are preliminarily registered into the apparatus as default scanning conditions with respect to each scan area, each clinical field, or each disease, and then a scan is performed under the scanning conditions every time.
However, because there are individual differences in the sizes of scan subjects (examination subjects), the FOV, the slice thickness, the slab thickness in a case of three-dimensional scanning, and the like need to be adjusted for each scan in accordance with the size of a scan subject, even if almost all of the other parameters can be kept constant. If the adjustment is not performed, aliasing may occur in an image, or a redundant area may be imaged, in some cases.
Although an adjustment of an FOV is often manually performed by an operator, a method of automatically setting an FOV by measuring the size of an examination subject with a system is proposed (for example, see JP-A H3-16851 (KOKOKU)). Usually, when the FOV is changed, a voxel size changes, so that the SN ratio of an image changes along with the change. Consequently, to keep the SN ratio constant, the operator needs to recover the SN ratio by correcting a parameter other than the changed FOV. Therefore, a method is proposed of predicting to what extent the SN ratio changes with respect to a change in a scanning parameter from the value of the scanning parameter before the change and notifying the operator of the predicted change in the SN ratio (for example, see JP-A 2004-357834 (KOKAI)).
However, when changing the FOV, it is not simply found out which parameter and to what extent needs to be changed in order to return the SN ratio to the original value from a change in the SN ratio arising from the change in the FOV. The reason for this is because such conditions vary depending on a functional direction of the FOV (whether a readout direction or a phase encoding direction).
For this reason, an operation of setting scanning conditions in which the same SN ratio is constantly maintained while changing the FOV requires a skill and an understanding of the principles of scanning according to the magnetic resonance imaging method. Such operation is substantially time consuming for an inexperienced operator, and sometimes results in a problem of an extension of a total examination time in some cases.